1. Field of the Invention
This invention relates generally to an active matrix display device, and particularly to a driving system for an active matrix display device which can change over between a display having an aspect ratio of 16:9 and another display having an aspect ratio of 4:3, and also to a horizontal shift register for driving an active matrix display device by line sequential horizontal driving.
2. Description of the Related Art
Development of active matrix display devices conforming to the high definition television standards has occurred in recent years. An active matrix display device conforming to the high definition television standards has such a horizontally elongated screen as shown in FIG. 5 and allows a display of the aspect ratio of 16:9 (such a display will be hereinafter referred to as a wide display). A large number of picture elements PXL are disposed in rows and columns on the screen so as to satisfy the aspect ratio of 16:9. Meanwhile, another active matrix display device having an ordinary screen size performs another display of the aspect ratio of 4:3 (such display will be hereinafter referred to as a normal display). In an active matrix display device of the high definition television standards, a structure which allows changing over between a wide display and a normal display is known. In order to perform a wide display with the active matrix display device of the high definition television standards, all picture element columns are used, as seen in FIG. 5. On the other hand, in order to effect a normal display, since a conversion in aspect ratio to 4:3 is involved, only those picture element columns which belong, for example, to a central area B shown in FIG. 5 are used to construct a display screen. In this instance, areas A and C on the opposite sides of the central area B are masked, for example, with side black.
Various means have been proposed for a changing over method between a wide display and a normal display. One of such methods involves compression processing of a video signal as illustrated in FIGS. 6(A) and 6(B). When a video signal for a normal display is input as seen from the waveform of FIG. 6(A), it is first converted by A/D conversion into digital data. In order to distribute the digital data only to the central area B of the horizontally elongated screen shown in FIG. 5, compression processing for each one horizontal period (1H) is performed at a desired ratio as seen from the waveform of FIG. 6(B). The digital data obtained by the compression processing is distributed to area B while fixed data for a masked display are supplied to the areas A and C (FIG. 5) on the opposite sides of the area B. However, the present compression processing method has a problem to be solved in that a digital processing circuit to be externally added is large in scale and has increased cost. Another structure has been proposed and is known wherein a mechanical shutter is disposed at each of the areas A and C at the opposite ends of the horizontally elongated screen in place of a masked display which is based on electronic processing. However, the alternative structure has another problem to be solved in that, since a physical mechanism is required, it is complicated in structure, which can be a cause of failure.